Many types of implantable medical devices (IMDs) have been clinically implanted over the last twenty years that deliver relatively high-energy cardioversion and/or defibrillation shocks to a patient's heart when a malignant tachyarrhythmia, e.g., atrial or ventricular fibrillation, is detected. Cardioversion shocks are typically delivered in synchrony with a detected R-wave when fibrillation detection criteria are met, whereas defibrillation shocks are typically delivered when fibrillation criteria are met and an R-wave cannot be discerned from the electrogram (EGM).
The current state of the art of ICDs or implantable pacemaker/cardioverter/defibrillators (PCDs) includes a full featured set of extensive programmable parameters which includes multiple arrhythmia detection criteria, multiple therapy prescriptions (for example, stimulation for pacing in the atrial, ventricular and dual chamber; atrial and ventricular for bradycardia; bi-atrial and/or bi-ventricular for heart failure; and arrhythmia overdrive or entrainment stimulation; and high level stimulation for cardioversion and/or defibrillation), extensive diagnostic capabilities and high speed telemetry systems. These full-featured ICDs or PCDs, hereinafter IMD, are typically implanted into patients who have had, and survived, a significant cardiac event such as sudden death. Additionally, these devices are expected to last up to 5-8 years and/or provide at least 200 life saving therapy shocks.
Even though there have been great strides in size reduction over the past 20 years, the incorporation of all these features in an IMD, including the longevity requirements, dictates that the devices be typically much larger than current state of the art pacemakers. Such devices are often difficult to implant in some patients (particularly children and thin, elderly patients) and typically require the sacrifice of 1 or 2 veins to implant the lead system because leads associated with implantation of an IMD utilize a transvenous approach for cardiac electrodes and lead wires. The defibrillator canister/housing is generally implanted as an active can for defibrillation and electrodes positioned in the heart are used for pacing, sensing and detection of arrhythmias.
Although IMDs and implant procedures are very expensive, most patients who are implanted have experienced and survived a sudden cardiac death episode because of interventional therapies delivered by the IMDs. Survivors of sudden cardiac death episodes are in the minority, and studies are ongoing to identify patients who are asymptomatic by conventional measures but are nevertheless at risk of a future sudden death episode. Current studies of patient populations, e.g., the MADIT II and SCDHeFT studies, are establishing that there are large numbers of patients in any given population that are susceptible to sudden cardiac death, that they can be identified with some degree of certainty and that they are candidates for a prophylactic implantation of a defibrillator (often called primary prevention). However, implanting currently available IMDs in all such patients would be prohibitively expensive. Further, even if the cost factor is eliminated there is shortage of trained personnel and implanting resources.
One option proposed for this patient population is to implant a prophylactic subcutaneous implantable cardioverter/defibrillator (SubQ ICD) such that when these patients receive a shock and survive a cardiac episode, they will ultimately have an implant with a full-featured ICD and transvenous leads.
While there are a few small populations in whom SubQ ICD might be the first choice of implantation for a defibrillator, the vast majority of patients are physically suited to be implanted with either an ICD or SubQ ICD. It is likely that pricing of the SubQ ICD will be at a lower price point than an ICD. Further, as SubQ ICD technology evolves, it may develop a clear and distinct advantage over ICDs. For example, the SubQ ICD does not require leads to be placed in the bloodstream. Accordingly, complications arising from leads placed in the cardiovasculature environment is eliminated. Further, endocardial lead placement is not possible with patients who have a mechanical heart valve implant and is not generally recommended for pediatric cardiac patients. For these and other reasons, a SubQ ICD may be preferred over an ICD.
There are technical challenges associated with the implantation of a SubQ ICD. For example, SubQ ICD sensing is challenged by the presence of muscle artifact, respiration and other physiological signal sources. This is particularly because the SubQ ICD is limited to far-field sensing since there are no endocardial or epicardial electrodes in a subcutaneous system. Further, sensing of atrial activation from subcutaneous electrodes is limited since the atria represent a small muscle mass and the atrial signals are not sufficiently detectable transthoracically. Thus, SubQ ICD sensing presents a bigger challenge than an ICD which has the advantage of electrodes in contact with the heart and, especially, inside the atrium. Accordingly, the design of a SubQ ICD is a difficult proposition given the technical challenges to sense and detect arrhythmias.
Yet another challenge could be combining a SubQ ICD with an existing pacemaker (IPG) in a patient. While this may be desirable in a case where an IPG patient may need a defibrillator, a combination implant of SubQ ICD and IPG may result in inappropriate therapy pace or shock by the SubQ ICD, due to inappropriate sensing of spikes from the IPG. Specifically, each time the IPG emits a pacing stimulus, the SubQ ICD may interpret it as a genuine cardiac beat. The result can be over-counting beats from the atrium, ventricles or both; or, because of the larger pacing spikes, sensing of arrhythmic signals (which are typically much smaller in amplitude) may be compromised.
Further, there may be patients who first receive a SubQ ICD and then develop bradycardia. This may occur with the use of beta-blockers, medical management of atrio-ventricular conduction due to development of atrial fibrillation or sinus node disease. Once patients have a SubQ ICD, it makes sense not to abandon the SubQ ICD system but leverage the SubQ ICD with a compatible IPG. Similarly, there may be an interest in patients who receive a SubQ ICD and then have an inappropriate shock. These patients may need an upgrade to an ICD, but they, too, could benefit from the use of a SubQ ICD compatible IPG.
Additionally, the implanting of two or more devices in a patient can be challenging with respect to programming and coordinating therapies delivered by the devices. Further, monitoring of the patient including the devices by use of conventional telemetry and diagnostics may pose additional burden on patient and device management resources. Additionally, several scenarios may arise in which an external defibrillator may be used on patients with an implantable SubQ ICD, a pacemaker, or both. A typical example is an emergency situation in which a patient with an IPG has collapsed and a rescue procedure is conducted. In this scenario, it is likely that an automatic external defibrillator (AED) may be used on the patient. It is therefore important that the IPG and the AED establish communications to coordinate therapy delivery activities. This includes arrhythmia detection, direction to charge/discharge each defibrillator, the delivery of a shock(s) and device protection. Specifically, the operations of one device may be suspended when one another device is providing a life support therapy.
Similarly, a SubQ ICD and an AED may cooperate to provide needed therapy. For example, if the SubQ ICD is not capable of restoring sinus rhythm, the AED may be given a chance to do it unaffected by the SubQ ICD. However, if the SubQ ICD is incapable of supplying sufficient energy, the combination of the SubQ ICD and the AED might be useful. In this setting, the SubQ ICD and AED could collaborate such that they shock simultaneously. The polarity must be coordinated such that the fields are additive or, alternatively, the concept of rotating fields may be Implemented. The AED patches could be positioned such that the first shock is delivered by one device and, after a further short delay, the second shock is delivered by one device and, after a short delay, the third shock is delivered with a slightly different orientation. This latter concept is well known in the art. With a SubQ ICD and an AED having two entirely separate sets of electrodes, it is feasible that the two could collaborate in this manner if they have inter-device communications. So it is envisioned that an AED could be in place in addition to a pacemaker and/or SubQ ICD. This could be a hospital setting such as a CCU or an ER. It could also be advanced life support as part of the emergency medical system with EMTs or paramedics.
Additionally, during cardioversion/defibrillation therapy delivery by either the AED or SubQ ICD, the IMD can take precautions to prevent damage due to high current flow and high voltage spikes.
Therefore, for these and other reasons, a need exists for a bi-directional communication system between an IPG and SubQ ICD, or alternatively, between an AED and an IPG and/or SubQ ICD. The IPG, by virtue of it having leads within the heart, should greatly improve the specificity of arrhythmia detection and allow additional therapy options, such as automatic tachycardia pacing (ATP). The IPG and SubQ ICD should be able to communicate wirelessly, either through RF or other intra-body communications medium.
When either device, the IPG or SubQ ICD detects the presence of another device it would go into a cooperative mode and operates accordingly. The IPG and the SubQ ICD, for example, should cooperate in such a way that the IPG would handle tachycardia detection and be in charge of directing charging and delivery of shocks. When the IPG detects a potentially shockable rhythm, it can direct the SubQ ICD to charge and then deliver a shock. If the rhythm might be pace terminable, the IPG can attempt ATP. During this time, it can direct the SubQ ICD to charge the capacitors and enters a stand by mode. Upon failure to convert the cardiac rhythm, the IPG would then direct the SubQ ICD to deliver a shock.
Conversely, the IPG should be in continuous communications with the SubQ ICD and anticipates the possibility that the SubQ ICD may issue a shock. At the time the SubQ ICD issues a shock, the IPG should protect itself and prepare for post-shock sensing and detection. If the IPG is unsure as to whether the rhythm is one requiring a shock, the IPG and SubQ ICD can perform a crosscheck to improve the confidence of arrhythmia detection.
Utilization of a SubQ ICD and IPG, may avoid the risk and morbidity associated with removal of an IPG to upgrade to an ICD, for example. In this case pacing leads may need to be removed or left in the vasculature while additional defibrillation leads should be implanted, thus crowding the veins. Alternatively, a SubQ ICD may be subcutaneously implanted and the IPG upgraded to communicate with the SubQ ICD. Thus, the need to replace the intracardiac pacing leads and the attendant risks could be eliminated.